The present invention relates to an exposure control system for cameras, cameras employing the exposure control system, and a method for controlling an exposure operation of cameras.
Conventionally, electronically controlled cameras have employed exposure control systems for measuring brightness of an object with use of a photometry system, and calculating an exposure value (Ev) based on the measured brightness. Based on the exposure value (Ev), an appropriate combination of exposure parameters, i.e., an f-number (Av: an aperture value) and a shutter speed (Tv: a time value) are determined.
Recently, a multi-point photometry system has been developed. The multi-point photometry system is capable of dividing an objective area into a plurality of measuring areas, and measuring the brightness for respective measuring areas. The exposure control system employing the multi-point photometry system is capable of determining the exposure value more appropriately compared with the exposure control system employing a single-point photometry system which have only one area.
In a camera which employs the exposure control system utilizing a multi-point photometry system, if a brightness range of an object is out of a latitude of a film, the exposure value is determined to have an average value so that a density or brightness of the entire image is well balanced. Alternatively, the exposure value is determined such that an image of an object having the lowest brightness corresponds to the lower limit of the latitude of the film, and let the image of the object having the higher brightness than upper limit of the latitude photographed as an over-exposed image.
In a camera that employs an exposure control system utilizing a single-point photometry, the exposure value should be determined to have a value corresponding to an average of brightness overall of the photometry area. In such a camera, when a photographing operation is performed in daytime, dark portions of a high-contrast object tend to be under-exposed since the bright portions of the object raise the average exposure value of the frame.
That is, due to the bright portions of the object, the brightness value Bv, which may be averaged brightness values of the dark and bright portions, is greater than the brightness value of the dark portions alone. Accordingly, for the dark portions, the obtained exposure value Ev may be greater than necessary. Therefore, the combination of the time value Tv and aperture value Av, which are determined based on such an exposure value, result in the under-exposed condition of the dark portions of the object.
To avoid such an under-exposed condition, it is detected whether the high-contrast condition, or a rear light (contrejour) condition is occurring, and exposure compensation is done. That is, the aperture size (i.e., the aperture value Av) and/or the shutter speed (i.e., the time value Tv) are decreased. However, over-compensation makes bright portions of the image over-exposed, and thus it is difficult to determine the appropriate compensation amount.
Conventionally, to photograph a relatively dark object with a bright background, a daylight synchro-flash photographing is executed.
It is preferable, when the daylight synchro-flash photographing is performed, the dark object is photographed with an appropriate exposed condition and the background is photographed as a bright image. If auxiliary illumination by the strobe is too strong, however, the photographs maybe over-exposed, and may include unnatural highlights and shadows. Thus, conventionally, it has been said that the auxiliary light should be made slightly weaker than required in order to take the appropriately exposed image of the dark object. If the contrast of an object is relatively high, however, dark portions of the object may not be sufficiently illuminated and remain dark, and the background may also be photographed as a dark image. When the multi-point photometry system is utilized, an improved exposure condition may be achieved than with the single-point photometry system. However, if an extremely bright object, such as the sun, is in the frame, the exposure control system determines the exposure condition mainly based on the extremely bright object. In such a case, the dark object may still remain under-exposed, and the background is also photographed as a dark image.